First step is to chose the right image for your content article.
Search and chose best descriptive images related with your text content article.
First step is to search for the best descriptive image After you chose or edit/create your image save it as .jpeg (.jpeg extension image format allows you to add some additional bites/info). If you use XP as OS, right Click on your image and chose Properties and fill out the empty fields like name, author, owner, keywords description and so on. On Windows 7 it's much simpler: right click > Properties and go to Details tab . Same step here...fill out all related fields (title, rated, tags, comment...and so on). Even if the jpeg image accumulates more bits this steps are important.
Image size and dimensions:
Image size is important also. Smaller images (favicons for example are great but not so good to use as relevant images in articles), bigger images slow down your site (page upload)...so..my choice will be this: jpeg image format (for high quality) smaller or maxim 500px width, for medium quality I use .gif images (poor quality image but less space, faster site upload/ page uploads faster).
Uploading and inserting images in article:
Do not use large images! If your article is about cars for example and you chose to upload/add a image to your article, DO NOT UPLOAD a wallpaper....even if looks great and unique, avoid this approach. Using large images when you only need a simple 200 or 450px wide image slows down your page speed. Tip: take time and crop your image at desired dimension. This step helps browser to display the actual original image without any re-dimensions or predefined image attributes."
ALWAYS use "alt" attribute (alternative text) and dimensions attribute (width="value in px" height="value in px"). If available use caption attribute.
Example: <img alt=" image description" src="image url source" width="200px" height="200px"/>
alt= informs browser what is the image description and often is displayed as text if image is not available.
width/height= informs browser about width/height image and how to display your image
Additional: align, caption, long description, hyperlink (linked to)
Avoid to use to many images hosted on other servers because http request slows down you speed page.